Leguminous crops such as soybeans, adzuki beans and kidney beans can be grown well even in soil with low nitrogen content by atmospheric nitrogen-fixation resulting from bacteroid formed in the root nodules that are symbiotically formed therein according to infection with root nodule bacteria.
Thus, in addition to utilization of ordinary fertilizers, leguminous crops are generally grown from seeds previously coated with root nodule bacteria.
In order to further accelerate growth of various leguminous crops and multiply crop yield thereof, a screening of root nodule bacteria strains appropriate for various leguminous crops has been strenuously conducted so far. Meanwhile, the development of root nodule bacteria transformed by gene introduction has been attempted. For example, JP-T-2000-514663 disclosed the root nodule bacteria that were modified with incorporation of the trifolitoxin gene and efficient in suppression of growth of other competent root nodule bacteria.
Because of their nitrogen-fixation ability, root nodule bacteria play key role in growth and yield of leguminous crops. In leguminous crops, for example, degree of dependence on root nodule bacteria in nitrogen absorption ranges from 30% to 65%, and is extremely high. Accordingly, root nodule bacteria having enhanced nitrogen-fixation ability are expected to greatly enlarge a possibility of cultivating leguminous crops in various soils or increasing yields thereof. In addition, such efficient nitrogen absorption of leguminous crops with root nodule bacteria necessarily decreases dependence on inorganic nitrogen fertilizers such as ammonium sulfate, and therefore contributes greatly toward reduction of agricultural cost and alleviation of environmental pollution.
However, root nodule bacteria whose nitrogen-fixation ability is enhanced to a practical extent have been provided neither by a screening of novel root nodule bacteria from the natural world nor by a production of modified root nodule bacteria through gene manipulation.
In the meantime, a catalase has an activity to catalyze decomposition of hydrogen peroxide acting harmfully on animal and plant individuals into water and oxygen. The inventors of this application previously isolated new Vibrio rumoiensis strain S-1 having quite a strong catalase activity (J. Ferment. Bioeng. 85:113–116, 1998), and further specified the catalase gene of this strain (JP-A-2000-316584; Appll. Environ. Microbiol. 65:67–72, 1999; J. Biosci. Bioeng. 90:530–534, 2000). However, it has been so far unknown that a nitrogen-fixation ability of root nodule bacteria is enhanced by a catalase activity. Besides, root nodule bacteria whose nitrogen-fixation ability is enhanced by introduction of a catalase gene have been absolutely unknown.
The invention of this application has been proposed by concerning above-mentioned circumstances, and aims to provide novel root nodule bacteria whose nitrogen-fixation ability is enhanced by gene introduction.